Sunday, July 21, 2013

To Build a Bridge Part IV: A Firm Foundation

As I mentioned before, the bridge is (in a very general sense: you skip around a lot) designed from the top down.  But it's built from the bottom up, and while the bridge-using public doesn't see it the foundation takes a fair bit of design.  This is where geotechnical engineers get into the game: they're the ones that calculate the strength of the soil and rock in the area.

Any project, once it's decided about where the bridge/wall is going to be, starts with some soil borings.  This essentially means take a drill rig and drilling into the earth some specified distance.  Every so often (1.5-5 feet it typical, depending on whose ordering it and how deep you are) a soil sample is taken and analyzed.  The main property being tested is the strength, which is inferred from multiple different measurements.

For a wall, a 30' (for those not "in the know", an apostrophe represents feet and a quotation is inches, so 30'-6" is 30 feet and 6 inches) boring is typical.  For a bridge, 100' is not uncommon.  Some footing types even require deeper borings, but that can get expensive quickly as equipment that goes that deep isn't cheap.

Piles sticking out of the ground for the West abutment foundation.  The piles have an estimated length they're supposed to be driven, but you don't stop until you hit a certain level of resistance.  Thus, most piles end up with a lot of extra length on them (you can see where the top of wall ends, the piles are supposed to be at about that elevation) and are cut off when it comes time to build the foundation around them.
 A "footing" is the part of the bridge that transfers the force from the structure to the soil.  A pile is a structural element that goes deep into the ground.  Above you can see steel, tube piles sticking out of the ground where the West abutment will go.  They terminate somewhere well beneath the surface.  The estimated lengths to "drive to capacity" are around 70' here.

Pile driving is exactly what it sounds like.  You take some amount of pile, stick it into the ground, and then continue to pound on it until it drives in deep enough to take the load.  Essentially you just put a pneumatic hammer on top of it and turn it on.  The hammer measures the resistance, and when you get enough you turn it off and start on the next pile.  They don't start with 70'+ long piles, they're much shorter.  As the pile starts to get close to running out of length, they pause the process and weld another length of pile onto the current one.

Drilling a concrete shaft for the pier footing
 But piles aren't the only way to support a structure.  This bridge actually started off with a "shallow foundation".  That essentially means something where you aren't drilling or driving, or doing more than excavating 5-10' down into the ground.  The original design called for a mat footing underneath all those columns that are going to be at the pier.  This essentially means digging out a place to put a 4'-5' deep concrete pad (as I recall, it was about 12' wide and spanned the whole, 136' of the bridge width from side to side).  That way, the weight from the bridge is supported entirely by pressure from the soil right there.

But we had to make a last minute change.  There's a 60" sewer pipe directly beneath the pier foundation.  Originally that was going to be grouted up (filled with a cementitious material: essentially turning it into a long, cylindrical rock) as it's old and a new one is coming to replace it.  But just before the project was bid, some guys in the utilities world decided they wanted to keep the pipe in use for another couple of years before closing it off.  We weren't sure the pipe could take the extra stress that would come from loading the soil on top of it (our shallow foundation) so we switched foundation types.

The edge of the wall can be seen in the foreground.  Most walls do not directly support the bridge, instead the force is transferred down behind the wall to the soils deep beneath it.
 To ensure accurate placement and sufficient strength, we went to drilled shafts.  These are shafts (here they're around 3.5'-4' in diameter) that are drilled into the ground to a specified depth (around 60-70').  Because nothing is being driven, you can't cut out early if you achieve capacity at a shallower depth than what's estimated.  Once the shaft is drilled it's filled with rebar and concrete, and you build the footing on top of it.
This specialized equipment was driven out from the Rockies, once the driller has done it's work, a rebar "cage", pre assembled steel bars, will be lowered into the hole and concrete poured in to fill it up.
If you can place your structure directly on bedrock you'd love to do so.  But in Southeast Wisconsin that's just not feasible (it's way too deep to reach) so the geotechnical engineers will give you a graph that tells you expected levels of support for a given pile type and size versus the depth you take it to.  It's then up to the structural engineer to pick the foundation type and design the pile if necessary (steel piles aren't designed: just counted, but the reinforcing in the drilled shafts have to be designed).

When you've built your foundation, you fill up all the excess area with sand and get started on the rest of the substructure.  The sand has all sorts of fancy names and criteria but it's essentially just sand.  Despite the parable, sand is actually a pretty good material to build on, but more importantly: it drains water.  You take a lot of precautions to avoid having water just sit right next to your concrete footing.  The concrete doesn't mind (as long as it doesn't freeze and thaw too much) but eventually the water penetrates and hits the rebar.  This is bad.  So you put quick-draining materials everywhere and then make sure there's a piece of perforated drain pipe somewhere nearby to take all that water away from your structure and get it out of there.

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